The present disclosure relates generally to the manufacturing of semiconductor devices, and more particularly to a photolithography process in semiconductor manufacturing.
Since the inception of the semiconductor industry, photolithography has been used for forming the components of integrated circuits. The continued increase in the density of components that can be placed on a chip has been largely due to advances in photolithography, and especially the ever decreasing wavelengths of radiation. As long as the critical dimension of the components is greater than the wavelength of the radiation used to expose the photoresist, advances in the art do not require any significant changes of the masks.
However, when the wavelength of the imaging radiation is larger than the critical dimension, the effects of diffraction, though always present, become sufficiently prominent to introduce noticeable distortions into the projected images. Those distortions are particularly sensitive to the distances between the various features in the image pattern and are frequently referred to as “proximity effects.”
Another problem associated with photolithography at wavelengths close to the critical dimensions is depth of focus (DOF). In particular, when the DOF is less than the thickness of the resist being exposed, image sharpness will be lost. In practice, because of diffraction effects, the resulting image often becomes a blurred circle.
When resolution is not a concern, DOF can be increased by restricting the incoming light to the center of the lens, thus reducing the angle of the light cone so that focused rays travel further before leaving the blurred circle. However, when resolution is also a consideration, that solution is no longer acceptable.
Traditionally, approaches for increasing DOF have been directed toward bringing both densely packed and isolated structures such as contact holes into simultaneous focus. However, since the increase of DOF for densely packed contact holes often result in the decrease of the DOF for isolated contact holes, such efforts frequently result in unfocused images.
To balance respective DOFs for densely packed and isolated contact holes, previously available art utilizes multiple masks with multiple exposures. For example, a first mask may be utilized for the densely packed contact holes, while a second mask may be utilized for the isolated contacted holes.
Since the variance of best focuses between masks is about 200 nm, while the DOF is about 300 nm for certain semiconductor devices, production defects may occur as a result. Thus, lithography engineers need to use the focus exposure matrix (FEM) method to identify the best focus for different masks. However, such approach is costly and timing consuming, and may cause delays in productions.
Therefore, it is desirable to adopt an improved system and method for lithography.